Yes, it is true that the fastest animals all have similar mitochondria. However, it is also true that all animals, fast or slow, have similar mitochondria. For example, human mitochondria have 16,569 base pairs, and encodes 13 proteins and 24 RNAs. In fact, most animal mitochondria are very similar in that they are all ~ 16kb in size and encode 13 proteins and 24 RNAs (http://nar.oxfordjournals.org/content/27/8/1767.short).